Allogeneic hematopoietic stem cell transplantation (aHSCT) is a potentially curative therapy used to treat several malignant diseases, of which the most frequent is acute myeloid leukemia (AML). As many as half of the ~8,000 aHSCTs performed in the U.S. each year will result in GVHD, thus graft-versus-host disease (GVHD) remains a significant cause of morbidity and mortality in patients receiving aHSCTs. Pre-transplant conditioning, including irradiation and chemotherapy for hematological tumors, causes widespread death of dividing cells, release of endogenous danger signals, and bacterial translocation due to gastrointestinal (GI) epithelial barrier dysfunction. These events promote the generation of a pro-inflammatory cytokine storm mediated by the activation of innate immune sensors which drives the differentiation and expansion of allo-reactive donor T cells. Activated donor anti-host T cells can then damage particular recipient tissues characteristic of GVHD, as well as mediate anti- tumor immunity targeted to both allogeneic and tumor-antigens. My previous findings demonstrate that Stimulator of Interferon Genes (STING), an innate immune sensor, promotes inflammation and GVHD following conditioning and MHC-matched murine aHSCT. The benefit of receiving an aHSCT is the accompanying ?graft- versus-leukemia? (GVL) response, mediated by donor T/NK cells to target and eradicate residual disease. Therefore, primary objectives of the F99 phase of this proposal are to identify how the STING pathway affects pre-clinical GVL responses, and if reduced GVHD in the absence of recipient STING can promote tumor vaccination strategies. Other strategies to reduce inflammatory responses to pre-transplant conditioning include the development of reduced intensity conditioning (RIC) regimens, which are less toxic than traditional chemoradiotherapy and lower the risks of both transplant-related mortality and GVHD but are limited in use due to the significantly increased risk of relapse after RIC. Since the median age of diagnosis for AML - the most frequent indication for aHSCT - is 68, frailty and the frequency of co-morbidities associated with elevated age often precludes the use of toxic myeloablative conditioning (MAC) regimens in many aHSCT recipients. As a result, studies during the K00 phase of this proposal will determine if multiple pre-clinical RIC regimens are associated with decreased activation of innate immune sensors, reduced GVHD and improved immune reconstitution after murine aHSCT. Experiments will also determine if prophylactic post-transplant antileukemic strategies targeting residual AML promotes comparable or improved anti-tumor immunity after RIC versus MAC. The studies in this proposal will provide new information regarding the involvement of STING and other innate sensors in the context of aHSCT. The long-term objective of my studies will be to develop new therapies that can be translated into the clinic to prevent or reduce GVHD without diminishing GVL, thereby increasing the overall number of potential transplant recipients to augment the use of aHSCT.